It is known in the art that fission neutrons, emitted from a single fission event, usually occur in groups of more than one neutron which are emitted at virtually the same instant in time. Other free neutrons, termed "singular neutrons" herein, occur individually and are distributed essentially randomly in time. For a given material to be analyzed the average number of neutrons per group is known. Essentially all free neutrons detected from a fission event will be detected within a relatively short period of time subsequent to the first neutron detected from that event. Various neutron counting circuits exist in the art which use this property to develop various indicative counts related to determining the number of fission events occurring in a sample.
The Gey et al patent (U.S. Pat. No. 3,225,200) discloses a device suitable for accumulating two counts, N.sub.1 and N.sub.2, related respectively to, (1) the total number of free neutrons being produced in a sample, and (2) the total number of fission events ocuring in the sample plus the number of singular neutrons occuring in the sample. The device disclosed by Gey et al generates the second of the above two mentioned counts by introducing a dead time element into the transmission line coupling the neutron detector to one of two counters. Essentially all neutrons related to a given fission event will be detected within a time period T subsequent to the first neutron detected from the event. The dead time in the circuit occurs subsequent to a detected neutron and is selected to embrace the aforesaid time period T. Thus, neutrons detected within T are not included in the count N.sub.2. It is assumed that a statistically insignificant number of free neutrons is missed by this arrangement. The number of fission events occuring may then be calculated by subtracting N.sub.2 from N.sub.1 and dividing that result by one less than the average number of neutrons emitted per fission event.
The Omohundro et al patent (U.S. Pat. No. 3,612,872) discloses a circuit likewise adapted to generate two neutron event counts, though the second of these counts differs from that developed by the Gey et al device. Specifically, a gross count is developed at a first counter. A second count is developed by delaying the pulse train of detected neutrons by a time period similar to T above, and counting such delayed pulses of that train which then occur coincidentally, or within a certain other time period T.sub.1 following gates generated by later detected neutrons. The patent teaches that this count registers non-fission event neutrons only. Substantially different first and second counts indicate a source as being fissionable.
Devices constructed according to the principle of the instant invention are intended for use in industrial environments. The samples to be analyzed typically comprise materials including small quantities of low level radioactive substances. Typical of such samples would be the ash of clothing which had been exposed to enriched uranium dioxide powder and then incinerated. In such ash the relative frequency of fission event related neutrons to other neutron events would be quite small. Events appearing like neutron events to an electrical counting circuit, such as static discharges, can also occur. In fact, the fission event related neutron count may be close to insignificant when compared to the gross number of events occuring.